Non-aqueous electrolyte type lithium ion secondary cell

ABSTRACT

There is provided a lithium ion secondary cell excellent in charging and discharging cycle characteristics. A lithium ion secondary cell includes: an electrode body including a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and a separator; and a non-aqueous electrolyte containing a lithium salt as a supporting salt in an organic solvent, the electrode body and the non-aqueous electrolyte being accommodated in a case. The positive electrode active material is a lithium transition metal oxide having a layered structure. The electrolyte contains a compound represented by a chemical formula (I) in an amount of β mol relative to the total content α mol of moisture to be mixed in the cell. β satisfies −1.3≦log(β/α)≦1.

TECHNICAL FIELD

The present invention relates to a lithium ion secondary cell suppressedin increase in internal resistance and having excellent charging anddischarging cycle characteristics.

BACKGROUND ART

A lithium ion secondary cell includes positive and negative electrodescapable of reversibly occluding and releasing lithium ions, and anelectrolyte interposed between both the electrodes. The lithium ions inthe electrolyte come and go between both the electrodes for performingcharging and discharging. The energy density is high. For this reason,use thereof has been also studied in the fields requiring alarge-capacity power source such as hybrid vehicles and electric cars.Representative examples of the electrolyte for use in a lithium ionsecondary cell may include a liquid electrolyte (non-aqueouselectrolyte) containing a lithium salt as a supporting salt in anon-aqueous solvent. Representative examples of the supporting salt mayinclude lithium slats including fluorine as a constituent element suchas lithium tetrafluoroborate and lithium hexafluorophosphate. PatentDocument 1 describes a technology regarding a lithium ion secondary cellusing such lithium salts.

-   Patent Document 1: Japanese Patent Application Laid-open No.    2005-32716

SUMMARY OF INVENTION

Incidentally, when the lithium salt including fluorine as a constituentelement as described above reacts with water, the lithium salt may bedecomposed to form hydrogen fluoride (HF). HF has a high corrosiveproperty to metals and inorganic oxides. Thus, when HF is formed in thecell, HF corrodes a collector. This and other factors increase theinternal resistance, which may cause degradation of the charging anddischarging cycle characteristics. For this reason, for the secondarycell using such lithium salts, it is important to prevent the hydrolysisreaction of the lithium salt for suppressing the formation of HF. PatentDocument 1 describes the following: to the electrolyte, a saltcontaining an anion having a dicarboxylic acid group is added as anadditive; as a result, the moisture in the cell is consumed, which cansuppress the formation of HF. Further, in Patent Document 1, there is adescription to the effect that at least a part of the additive isdecomposed by a charging treatment, and that the decomposed products aredeposited on the surfaces of the positive electrode and/or the negativeelectrode, but such decomposed products is low in resistance, andstable.

However, a study by the present inventors indicates as follows: use ofthe additive reversely causes an increase in internal resistance (anincrease in initial internal resistance and/or an increase in internalresistance (endurance resistance) due to repetition of the charging anddischarging cycle); this may degrade the charging and discharging cyclecharacteristics.

An object of the present invention is to provide a lithium ion secondarycell effectively suppressed in increase in internal resistance andimproved in charging and discharging cycle characteristics by the use ofthe additive as described above. Further, another object thereof is toprovide a method for manufacturing such a lithium ion secondary cell.

The present inventors focused attention on the decomposed products ofthe additive deposited on the negative electrode surface as a factorwhich may still cause an increase in internal resistance even when theformation of HF is suppressed by the additive. Then, it has beenrevealed as follows: the decomposed products cause an increase innegative electrode resistance, so that use of the additive may reverselyresult in an increase in internal resistance. Thus, the presentinventors found a technology capable of suppressing the formation of HFusing the additive, and implementing a suppressing effect of an increasein internal resistance, and completed the present invention.

The present invention provides a lithium ion secondary cell, including:an electrode body including a positive electrode having a positiveelectrode active material, a negative electrode having a negativeelectrode active material, and a separator, as constituent members; anda non-aqueous electrolyte containing a lithium salt as a supporting saltin an organic solvent, the electrode body and the non-aqueouselectrolyte being accommodated in a case. The positive electrode activematerial contains a layered oxide having lithium and a transition metalas a main component. Herein, the layered oxide means an oxide having alayered crystal structure including two different stacked layers.Further, the supporting salt is a lithium salt containing fluorine as aconstituent element (which will be hereinafter also referred to as afluorine-containing lithium salt). Herein, the lithium salt containingfluorine as a constituent element means a lithium salt in general foruse as a supporting salt for a common lithium ion secondary cell, andcapable of reacting with water to form HF. The electrolyte furthercontains, as an additive, a compound represented by the followingchemical formula (I):

Herein, the cell is configured such that the case accommodates thereinthe additive in a number of moles of β satisfying the followingmathematical expression (A):

[Mathematical Expression 1]

−1.3≦log(β/α)≦1  (A)

where α represents the total number of moles of moisture to be mixed inthe case when the cell is assembled.

With such a configuration, the compound represented by the chemicalformula (I) (which may be hereinafter referred to as an additive (I))consumes moisture in the cell. This can suppress the formation of HF dueto the reaction between the fluorine-containing lithium salt andmoisture. Then, the amount of the additive (I) to be accommodated in thecase is set so as to be commensurate with the amount of moisture to bemixed into the case. Accordingly, it is possible to avoid the followingsituation: the amount of the additive becomes insufficient, which makesit impossible to completely prevent the formation of HF (as a result,the endurance resistance increases). In addition, it is possible tosuppress the detrimental effect (increase in initial resistance) due tothe excess of the additive. Therefore, it is possible to provide alithium ion secondary cell suppressed in increase in internal resistanceand excellent in charging and discharging cycle characteristics.

In another preferable aspect, the supporting salt is at least oneselected from LiPF₆, LiBF₄, LiAsF₆, and LiSbF₆.

Such a supporting salt is relatively high in solubility in an organicsolvent and lithium ion conductivity. The use thereof with the additivein the aspect herein disclosed suppresses the hydrolysis and the HFformation resulting therefrom. For this reason, the supporting salt canbe preferably used for the lithium ion secondary cell of the presentinvention.

As a still other aspect of the present invention, there is provided amethod for manufacturing any lithium ion secondary cell hereindisclosed. The method includes the following steps of:

(v) a step of preparing the constituent members of the electrode body;

(w) a step of ascertaining the molar equivalent value α of the totalcontent of moisture which can be mixed in the case;

(x) a step of substituting a value of the α into the mathematicalexpression (A), and calculating a numerical value range of the βsatisfying the mathematical expression (A);

(y) a step of preparing the non-aqueous electrolyte, the concentrationof the additive in the electrolyte being determined so that theelectrolyte accommodated in the case contains the additive in a numberof moles within the numerical value range of the α calculated in thestep (x); and

(z) a step of assembling a lithium ion secondary cell using theconstituent members of the electrode body and the electrolyte.

In accordance with this specification, as a preferred aspect of themethod, there is provided a method for manufacturing a lithium ionsecondary cell that includes the following steps:

(a) a step of preparing the positive electrode, the negative electrode,the separator, and an initial electrolyte serving as constituent membersof the cell, respectively, the initial electrolyte including at least afluorine-containing lithium salt as a supporting salt dissolved in anorganic solvent in a prescribed concentration;

(b) a step of ascertaining, based on the content of moisture containedin the positive electrode, the negative electrode, the separator, andthe initial electrolyte prepared in the step (a), a molar equivalentvalue α of the total content of moisture to be mixed in the case due tothe constituent members;

(c) a step of substituting a value of the α into the mathematicalexpression (A), and calculating a numerical value range of the βsatisfying the mathematical expression (A);

(d) a step of adding the additive in a number of moles within thenumerical value range of β calculated in the step (c) to the initialelectrolyte, and preparing a final electrolyte; and

(e) a step of assembling a lithium ion secondary cell under anenvironment with a dew point of −10° C. or less using the positiveelectrode, the negative electrode, the separator, and the finalelectrolyte.

Herein, in the step (a), the positive electrode, the negative electrode,the separator, and the initial electrolyte are prepared, respectively.Alternatively, the previously prepared ones are obtained (purchased orthe like).

In the step (b), the contents of moisture present (deposited, dissolved,or the like) on respective members (the positive electrode, the negativeelectrode, the separator, and the initial electrolyte) of the cell areascertained, respectively. The total is determined as the total contentof moisture to be mixed in the case.

In the step (c), the amount of the additive to be used commensurate withthe total moisture content α mol ascertained in the step (b) is referredto as β mol. The value of α is substituted into the mathematicalexpression (A) to calculate the numerical value range of β.

In the step (d), within the numerical value range of β calculated in thestep (c), the amount (number of moles) of the additive to be used isappropriately selected. The additive in a mass commensurate therewith isadded to the initial electrolyte, thereby preparing a final electrolyte.

In the step (e), in an environment in which external moisture mixingscarcely occurs such as that with a dew point of −10° C. or less, theelectrode body is formed using the positive electrode, the negativeelectrode, and the separator. The electrode body and the finalelectrolyte are accommodated in a cell case, or the like. Then, the caseis subjected to sealing or the like. Thus, a lithium ion secondary cellis assembled.

With such a method, the moisture in the cell is consumed by the additive(I), which can suppress the formation of HF due to the reaction betweenthe fluorine-containing lithium salt and moisture. Then, the amount ofthe additive (I) to be accommodated in the case is set so as to becommensurate with the amount of moisture to be mixed into the case.Accordingly, it is possible to avoid the following situation: the amountof the additive becomes insufficient, which makes it impossible tocompletely prevent the formation of HF (as a result, the enduranceresistance increases). In addition, it is possible to suppress thedetrimental effect (increase in initial resistance) due to the excess ofthe additive. Therefore, it is possible to provide a lithium ionsecondary cell suppressed in increase in internal resistance andexcellent in charging and discharging cycle characteristics.

Further, the lithium ion secondary cell herein disclosed has beenreduced in initial internal resistance, and can also be suppressed inincrease in internal resistance due to repetition of the charging anddischarging cycle, and hence is preferable as an onboard cell requiredto have excellent charging and discharging cycle characteristics.Therefore, in accordance with the present invention, as a still furtheraspect, there is provided a vehicle including the lithium ion secondarycell herein disclosed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic perspective view showing a cell in accordance withone embodiment;

FIG. 2 is a schematic plan view showing positive and negative electrodesand a separator forming the cell in accordance with one embodiment;

FIG. 3 is a cross-sectional view along line of FIG. 1;

FIG. 4 is a graph plotting the initial internal resistance of the celland the internal resistance increase rate after a charging anddischarging cycle test in terms of the relationship between the additiveamount and the total moisture content;

FIG. 5 is a side view schematically showing a vehicle (car) including alithium ion secondary cell of the present invention; and

FIG. 6 is a perspective view illustrating a schematic shape of a 18650model lithium ion secondary cell.

DESCRIPTION OF EMBODIMENTS

Below, preferred embodiments of the present invention will be described.Incidentally, matters necessary for carrying out the present invention,except matters specifically referred to in the present specification,can be grasped as design matters of those skilled in the art based onthe related art in the field. The present invention can be carried outbased on the contents disclosed in the present specification andtechnical common sense in the field.

The technology herein disclosed is applicable to a lithium ion secondarycell which includes an electrode body including a positive electrodecontaining a lithium transition metal oxide having a layered structure(which may be hereinafter also referred to as a layered lithiumtransition metal oxide or simply as a layered oxide) as a positiveelectrode active material, and a negative electrode containing anegative electrode active material, and a separator as constituentmembers, and a non-aqueous electrolyte containing a fluorine-containinglithium salt as a supporting salt in an organic solvent. The outsideshape of the secondary cell can be appropriately changed according tothe intended uses, and has no particular restriction. However, theoutside shape may be an outside shape such as a rectangularparallelepiped-like shape, a flat shape, or a cylindrical shape.Further, the shape of the electrode body can vary according to the shapeof the secondary cell and the like, and hence has no particularrestriction. For example, there can be preferably adopted an electrodebody including sheet-like positive electrode and negative electrodewound together with sheet-like separators. Below, the present inventionwill be described more specifically with a lithium ion secondary cell ofsuch an embodiment as an example. However, the applicable objects of thepresent invention are not limited to such cells and manufacturingthereof.

As a preferable applicable object of the technology herein disclosed,mention may be made of a lithium ion secondary cell using an electrolytecontaining a supporting salt capable of reacting with water and formingHF. Specific examples of such a supporting salt may include LiPF₆,LiBF₄, LiAsF₆, and LiSbF₆. The lithium salts have a relatively highlithium ion conductivity, and hence are preferable. Out of these, LiPF₆and LiBF₄ can be preferably used because relatively low-priced andhigh-purity commercially available products are available. Theconcentration of the supporting salt in the electrolyte has noparticular restriction, and, for example, can be set equal to theconcentration of the electrolyte for use in a conventional lithium ionsecondary cell. Generally, there can be preferably used a non-aqueouselectrolyte containing a supporting salt in a concentration of about 0.1mol/L to 5 mol/L (e.g., about 0.8 mol/L to 1.5 mol/L).

Further, as the organic solvents (non-aqueous solvents) to be used forthe non-aqueous electrolytes, there can be preferably used aproticsolvents such as carbonates, esters, ethers, nitriles, sulfones, andlactones. There can be used organic solvents commonly used for a lithiumion secondary cell, such as ethylene carbonate (EC), propylene carbonate(PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethylcarbonate (EMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane,tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane,diethylene glycol dimethyl ether, ethylene glycol dimethyl ether,acetonitrile, propionitrile, nitromethane, N,N-dimethylformamide,dimethyl sulfoxide, sulfolane, and γ-butyrolactone (BL), independentlyalone or in combination of two or more thereof.

With the technology herein disclosed, the non-aqueous electrolytecontains, in addition to the supporting salt (fluorine-containinglithium salt), a compound expressed by the chemical formula (I) in aproper amount as an additive. The amount of the additive used is setwithin the range capable of effectively implementing reduction of theinitial resistance and suppression of an increase in internalresistance. The range can be calculated from the total moisture contentwhich can be mixed into the cell manufactured. Therefore, with thetechnology herein disclosed, there is prepared an initial electrolyteprepared so as to contain the supporting salt in a desired concentrationin an organic solvent. Thus, the total content of moisture which may bemixed into the cell is ascertained. Then, an additive in an amountcommensurate with the total moisture content is added to the initialelectrolyte to prepare a non-aqueous electrolyte (final electrolyte).

Generally, a lithium ion secondary cell to be the applicable object ofthe technology herein disclosed is assembled in a highly driedenvironment (e.g., with a dew point of −10° C. or less (typically −10 to−50° C.), preferably a dew point of −20 to −50° C., and more preferablya dew point of about −30° C. to −50° C.). For this reason, mixing ofmoisture from the environment is to such a degree as to hardly exert aninfluence. Thus, mixing of moisture into the cell can be regarded as theone mainly brought by the members (typically, the positive electrode,the negative electrode, the separator, and the electrolyte) forming thecell. Therefore, the total content a mol of moisture which may be mixedfrom the members into the cell can be determined in the followingmanner. The moisture contents (mol) of respective members in amountsused for cell manufacturing are ascertained, and these are summed up.The moisture content of each of the members is typically calculated froma value obtained by quantifying the moisture content (mol/g) per unitmass present on (deposited on, dissolved in, or the like) the member.

As the method for quantifying the moisture content per unit mass of eachmember, there is used a method capable of quantifying a trace amount ofmoisture. For example, the Karl Fischer's method can be preferablyadopted. For example, a sample is collected from each member of apositive electrode sheet, a negative electrode sheet, a separator, andan electrolyte (initial electrolyte). The moisture content contained inthe sample is quantified by means of a moisture meter (Karl Fischermoisture meter, or the like). Thus, the moisture content (mol/g) perunit mass of the member can be determined.

Also for other members (a cell case, a terminal, and the like) than themembers, the moisture contents can be similarly quantified. However,when the surface areas of other members such as the cell case and theterminal are remarkably smaller relative to the surface areas of themembers (the positive electrode, the negative electrode, and theseparator), as with, for example, the wound electrode body, the moisturecontents of the other members can be omitted to be added to the totalmoisture content as the one scarcely exerting influences. Similarly, themoisture content adsorbed on the collector exposed part of eachelectrode sheet is also generally negligible.

Incidentally, the member of which the moisture content per unit mass wasquantified is stored in an environment in which further moisture mixingis substantially not caused. Then, at this step, another cell ismanufactured using a part of the member, the moisture content per unitmass quantified before storage may be adopted in order to ascertain themoisture content of the member. Namely, when the moisture content of themember is kept roughly constant (e.g., when the members are produced inthe same lot, and stored in an environment in which external moisturemixing hardly can occur), the moisture content per unit mass is oncequantified. Then, in subsequent cell manufacturing, the same value canbe adopted to ascertain the moisture content of the member. Namely, insecond or later cycle of cell manufacturing, the quantification of themoisture content per unit mass can be appropriately omitted. The samealso applies to the case where the content of moisture to be broughtinto the case can be predicted (ascertained) from the information suchas the past actual results.

With the technology herein disclosed, the amount of the additive (I) tobe added to the non-aqueous electrolyte can be appropriately selectedwithin the range of the numerical value of β (mol) determined bysubstituting the total moisture content a calculated in the foregoingmanner into the mathematical expression (A). Herein, the commonlogarithm (log(β/α)) of the value (β/α) of the mole ratio of the amountof the additive used to the total moisture content is roughly within therange of −1.3 to +1, and preferably can be set within the range of −0.5to +0.6. The value of β is determined based on the value of α so thatthe log(β/α) falls within the range. This suppresses an increase ininitial internal resistance, which can also further suppress an increasein internal resistance (endurance resistance) due to repetition of thecharging and discharging cycle. When the value of the log(β/α) is toosmaller than the foregoing range, the HF formation suppressing effectcannot be obtained sufficiently. As a result, the endurance resistancemay increase. Whereas, when the value is too larger than the foregoingrange, the amount of decomposed products deposited on the negativeelectrode surface increases. As a result, the internal resistance(initial internal resistance) may increase.

Below, an embodiment of a lithium ion secondary cell including the woundelectrode body will be further specifically described by reference tothe schematic views shown in FIGS. 1 to 3. As shown, a lithium ionsecondary cell 10 in accordance with the present embodiment includes acase 12 made of a metal (also preferably made of a resin or made of alaminate film). In the case 12, there is accommodated a wound electrodebody 20 formed by stacking a long positive electrode sheet 30, aseparator 50A, a negative electrode sheet 40, and a separator 50B inthis order, and then winding them in a flat form.

The positive electrode sheet 30 can be manufactured by, for example,coating and drying a positive electrode mixture on at least one side(preferably both sides) of a positive electrode collector 32, andforming a positive electrode mixture layer 35. As the positive electrodemixture, there can be used a paste-like or slurry-like compositionobtained by dispersing a layered oxide as the positive electrode activematerial in a proper solvent, if required, together with a conductivematerial, a binder, and the like. As the positive electrode collector32, there is preferably used a conductive member including a metal withgood conductivity. For example, there can be preferably used aluminum oran alloy including aluminum as the main component. The shape of thepositive electrode collector can vary according to the shape of thelithium ion secondary cell or the like, and hence has no particularrestriction. The shapes may be various forms such as rod form, plateform, sheet form, foil form, and mesh form. In the present embodiment, asheet-like positive electrode collector can be preferably adopted.

As the layered oxide, there can be appropriately selected and used anoxide having a composition including lithium and a transition metal, andhaving a layered structure. For example, preferred is use of one, or twoor more layered oxides selected from layered lithium nickel type oxidesand layered lithium cobalt type oxides.

Herein, the “layered lithium nickel type oxides” are intended toembrace, other than layered oxides including Li and Ni as constituentmetal element, even composite oxides each including other one, or two ormore metal elements than Li and Ni (i.e., transition metal elementsother than Li and Ni and/or main group metal elements) in a contentequal to or smaller than that of Ni in terms of number of atoms(typically, a content smaller than that of Ni; when two or more metalelements other than Li and Ni are included, a content equal to orsmaller than that of Ni, or smaller than that of Ni for any of them),and having a layered structure with stability. Such metal elements canbe one, or two or more elements selected from the group consisting of,for example, Co, Al, Mn, and Mg. Similarly, the “layered lithium cobalttype oxides” are intended to embrace even composite oxides eachincluding other one, or two or more metal elements than Li and Co in acontent equal to or smaller than that of Co (typically, a contentsmaller than that of Co), and having a layered structure with stability.As particularly preferred positive electrode active materials, there areexemplified layered oxides including three kinds of transition metalelements (which is also referred to as tertiary lithium oxides).

As such layered lithium transition metal oxides, there can be used, forexample, those manufactured or provided with conventional known methodsas they are.

As the conductive materials, there are preferably used conductive powdermaterials such as carbon powders and carbon fibers. As the carbonpowders, preferred are various carbon blacks such as acetylene black,furnace black, Ketjen black, and graphite powder. The conductivematerials can be used independently alone, or in combination of two ormore thereof. The amount of the conductive material included in thepositive electrode mixture may be appropriately selected according tothe kind and the amount of the positive electrode active material.

The binders can be appropriately selected from, for example,water-soluble polymers dissolving in water, polymers dispersed in water,polymers dissolving in a non-aqueous solvent (organic solvent), and thelike to be used. Further, these may be used independently alone, or maybe used in combination of two or more thereof.

Examples of the water-soluble polymers may includecarboxymethylcellulose (CMC), methylcellulose (MC), cellulose acetatephthalate (CAP), hydroxypropyl methylcellulose (HPMC), hydroxypropylmethylcellulose phthalate (HPMCP), and polyvinyl alcohol (PVA).

Examples of the water-dispersible polymers may includefluorine-containing resins such as polytetrafluoroethylene (PTFE),tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA),tetrafluoroethylene-hexafluoropropylene copolymer (FEP), andethylene-tetrafluoroethylene copolymer (ETFE), vinyl acetate copolymer,styrene butadiene block copolymer (SBR), acrylic acid-modified SBR resin(SBR type latex), and rubbers such as gum arabic.

Examples of the polymers dissolving in a non-aqueous solvent (organicsolvent) may include polyvinylidene fluoride (PVDF), polyvinylidenechloride (PVDC), polyethylene oxide (PEO), polypropylene oxide (PPO),and polyethylene oxide-propylene oxide copolymer (PEO—PPO).

The amount of the binder to be added may be appropriately selected fromthe type and the amount of the positive electrode active material.

Whereas, the negative electrode sheet 40 can be manufactured by, forexample, coating and drying a negative electrode mixture on at least oneside (preferably both sides) of a negative electrode collector 42, andforming a negative electrode mixture layer 45. As the negative electrodemixture, there can be used a paste-like or slurry-like compositionobtained by dispersing a negative electrode active material in a propersolvent, if required, together with a binder, and the like. As thenegative electrode collector 42, there is preferably used a conductivemember including a metal with good conductivity. For example, there canbe used copper or an alloy including copper as the main component. Theshape of the negative electrode collector 42 can vary according to theshape of the lithium ion secondary cell or the like, and hence has noparticular restriction. The shapes may be various forms such as rodform, plate form, sheet form, foil form, and mesh form. In the presentembodiment, the sheet-like negative electrode collector 42 made ofcopper is used, and can be preferably used for the lithium ion secondarycell 10 including the wound electrode body 20.

As the negative electrode active materials, one, or two or more ofmaterials conventionally used for a lithium ion secondary cell can beused without particular restriction. For example, as preferred negativeelectrode active materials, carbon particles may be mentioned. There canbe preferably used particulate carbon material (carbon particles)including a graphite structure (layered structure) at least in a partthereof. There can be preferably used any carbon material of a so-calledgraphite one (graphite), a hardly graphitizable carbonaceous one (hardcarbon), and easily graphitizable carbonaceous one (soft carbon), andthe one having a combined structure thereof. Out of these, particularly,graphite particles of natural graphite and the like can be preferablyused.

For the binder, the same ones as those for the positive electrode can beused independently alone, or in combination of two or more thereof.Although not particularly restricted, the amount of the binder to beused per 100 parts by mass of the negative electrode active material canbe set, for example, within the range of 0.5 to 10 parts by mass.

Further, as the separators 50A and 50B to be used in a manner stackedwith the positive electrode sheet 30 and the negative electrode sheet40, there can be preferably used a porous film including a polyolefintype resin such as polyethylene or polypropylene. The film may be amonolayer or a multilayer. For the two sheets of separators 50A and 50B,the same one may be used, or different ones may be used.

As shown in FIG. 2, at a first end along the longitudinal direction ofthe positive electrode sheet 30, the positive electrode mixture layer 35is not formed (or has been removed after formation), so that thepositive electrode collector 32 is exposed. Also for the negativeelectrode sheet 40, similarly, at a first end thereof, the negativeelectrode collector 42 is exposed. The positive and negative electrodesheets 30 and 40 are stacked with the separators 50A and 50B to form alaminate. At this step, the positive and negative electrode sheets 30and 40 are stacked in slight misalignment therebetween so that the firstend (positive electrode collector exposed part) of the positiveelectrode sheet and the first end (negative electrode collector exposedpart) of the negative electrode sheet are disposed symmetrically withrespect to the axis in the longitudinal direction of the laminate, andso that both the mixture layers 35 and 45 are stacked one on another.The laminate is wound. Then, the resulting wound body is crushed fromthe side direction to be flattened. This results in the flat-shapedwound electrode body 20.

The resulting wound electrode body 20 is accommodated in the case 12(FIG. 3). In addition, electric connections are established between theexposed part of the positive electrode collector 32 and an externalconnection positive electrode terminal 14, and between the exposed partof the negative electrode collector 42 and an external connectionnegative electrode terminal 16, respectively. At this step, it isconfigured such that the terminals are partially disposed outside thecase 12. Then, the non-aqueous electrolyte (final electrolyte) isdisposed (injected) into the case 12. The opening of the case 12 issealed by welding between the case and a lid member 13 correspondingthereto, or the like. Thus, assembly of the lithium ion secondary cell10 is completed. Incidentally, sealing of the case 12 and disposition ofthe electrolyte can be performed in the same manner as with the methodperformed in manufacturing of a conventional lithium ion secondary cell.

The lithium ion secondary cell in accordance with the present inventionhas excellent charging and discharging cycle characteristics asdescribed above. For this reason, the secondary cell can be preferablyused as a powder source for a motor (electric motor) to be mounted in avehicle such as a car. Such secondary cells may also be used in the formof an assembled battery including a plurality of them connected inseries and/or in parallel. Therefore, the present invention provides, asschematically shown in FIG. 5, a vehicle (typically, a car,particularly, a car including an electric motor such as a hybrid car, anelectric car, or a fuel cell car) 1 including such a lithium ionsecondary cell (which may be in the form of an assembled battery) 10 asa power source.

Below, a description will be given to some examples regarding thepresent invention. However, it is not intended that the presentinvention be limited to such specific examples.

Example 1 Preparation of Cell Member

As a positive electrode mixture, a positive electrode active material,carbon black (CB) as a conductive material, and PVDF as a binder weremixed with N-methyl-2-pyrrolidone (NMP) so that the mass ratio of thematerials was 85:10:5, and so that the solid content concentration (NV)was roughly 50 mass %. As a result, a slurry-like composition wasprepared.

Herein, as a positive electrode active material, there was used apowder-like lithium-nickel-cobalt-manganese type oxide(LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂) with an average particle diameter of 8μm, a specific surface area of 1.0 m²/g, and a theoretical dischargecapacity of about 140 mA/g. The oxide has a layered crystal structure.Further, as the CB, acetylene black (AB) was used.

The positive electrode mixture was coated and dried on both the sides oflong aluminum foil (positive electrode collector) with a thickness ofabout 15 μm. The coating amount (in terms of NV) of the positiveelectrode mixture was adjusted so as to be about 160 g/m² in total forboth sides. After drying, pressing was performed so that the overallthickness was about 80 μm, resulting in a positive electrode sheet.

As the negative electrode mixture, a natural graphite power, SBR, andCMC were mixed with ion exchange water so that the mass ratio of thematerials was 98:1:1, and so that NV was 45 mass %. As a result, aslurry-like composition was prepared. The negative electrode mixture wascoated and dried on both the sides of long copper foil (negativeelectrode collector) with a thickness of about 10 μm. The coating amount(in terms of NV) of the negative electrode mixture was adjusted so as tobe about 80 g/m² in total for both sides. After drying, pressing wasperformed so that the overall thickness was 65 μm, resulting in anegative electrode sheet.

The non-aqueous electrolyte (initial electrolyte) was prepared bydissolving LiPF₆ as a supporting salt in a mixed solvent containing EC,DMC, and EMC in a volume ratio of 1:1:1 in a concentration of 1 mol/L.

As the separators, there were prepared two long porous polyethylenesheets with a thickness of 20 μm.

[Quantification of Total Moisture Content]

Each moisture content per unit area was quantified for the positiveelectrode sheet, the negative electrode sheet, the electrolyte, and theseparator was quantified in the foregoing manner. Incidentally, assamples, there were used 1 g of the positive electrode, 1 g of thenegative electrode, 1 g of the electrolyte, and 0.2 g of the separator.For the moisture meter, there was used Karl Fischer moisture meter(model “AQ-7” manufactured by HIRANUMA SANGYO Co., Ltd.).

The total content of moisture which can be mixed into a 18650 model celldescribed later (i.e., the total moisture content of respective membersfor use in manufacturing of the cell) was calculated from the quantifiedvalues. As a result, the total moisture content was 0.01111 mol. Herein,the amounts of respective members for use in manufacturing of a cell 100were 7 g for the positive electrode sheet, 7 g for the negativeelectrode sheet, 1 g for the (two) separators, and 8 g for the initialelectrolyte.

[Preparation of Final Electrolyte]

To the initial electrolyte, was added lithium bis(oxalato)borate (LiBOB)(the compound of the chemical formula (I)) in an amount of 0.00060 mol(0.116 g) as an additive, thereby to prepare a final electrolyte.

[Manufacturing of Cell]

Using the members, a 18650 model (cylinder type 18 mm in diameter and 65mm in height) lithium ion secondary cell 100 (see FIG. 6) wasmanufactured in the following procedure.

Namely, the positive electrode sheet and the negative electrode sheetwere stacked together with the two sheets of separators as shown in FIG.2. The laminate was wound in the longitudinal direction to manufacture awound electrode body. The electrode body was accommodated together withthe final electrolyte in a cylindrical container. The container wassealed to form the lithium ion secondary cell 100. Then, as an initialcharging treatment, an operation of performing 3-hour constant currentcharging at a rate of 1/10 C, and then, performing charging up to 4.1 Vat a rate of ⅓ C and an operation of performing discharging down to 3.0V at a rate of ⅓ C were repeated three times. As a result, a cell inaccordance with Example 1 was obtained.

Example 2

A positive electrode sheet, a negative electrode sheet, an initialelectrolyte, and separators were prepared in the same manner as inExample 1. The total content of moisture which can be mixed into a cellwas quantified, and was found to be 0.00278 mol. In the present example,to the initial electrolyte, was added LiBOB in an amount of 0.00090 mol(0.174 g), thereby to prepare a final electrolyte.

Using the members, a cell was formed, and an initial charging treatmentwas performed in the same manner as in Example 1. As a result, the cellin accordance with Example 2 was obtained.

Example 3

A positive electrode sheet, a negative electrode sheet, an initialelectrolyte, and separators were prepared in the same manner as inExample 1. The total moisture content was quantified, and was found tobe 0.00167 mol. In the present example, to the initial electrolyte, wasadded LiBOB in an amount of 0.00196 mol (0.380 g), thereby to prepare afinal electrolyte.

Using the members, a cell was formed, and an initial charging treatmentwas performed in the same manner as in Example 1. As a result, the cellin accordance with Example 3 was obtained.

Example 4

A positive electrode sheet, a negative electrode sheet, an initialelectrolyte, and separators were prepared in the same manner as inExample 1. The total moisture content was quantified, and was found tobe 0.00111 mol. In the present example, to the initial electrolyte, wasadded LiBOB in an amount of 0.00392 mol (0.760 g), thereby to prepare afinal electrolyte.

Using the members, a cell was formed, and an initial charging treatmentwas performed in the same manner as in Example 1. As a result, the cellin accordance with Example 4 was obtained.

Example 5

A positive electrode sheet, a negative electrode sheet, an initialelectrolyte, and separators were prepared in the same manner as inExample 1. The total moisture content was quantified, and was found tobe 0.00067 mol. In the present example, to the initial electrolyte, wasadded LiBOB in an amount of 0.00588 mol (1.14 g), thereby to prepare afinal electrolyte.

Using the members, a cell was formed, and an initial charging treatmentwas performed in the same manner as in Example 1. As a result, the cellin accordance with Example 5 was obtained.

Example 6

A positive electrode sheet, a negative electrode sheet, an initialelectrolyte, and separators were prepared in the same manner as inExample 1. The total moisture content was quantified, and was found tobe 0.02778 mol. In the present example, to the initial electrolyte, wasadded LiBOB in an amount of 0.00030 mol (0.0581 g), thereby to prepare afinal electrolyte.

Using the members, a cell was formed, and an initial charging treatmentwas performed in the same manner as in Example 1. As a result, the cellin accordance with Example 6 was obtained.

Example 7

A positive electrode sheet, a negative electrode sheet, an initialelectrolyte, and separators were prepared in the same manner as inExample 1. The total moisture content was quantified, and was found tobe 0.00028 mol. In the present example, to the initial electrolyte, wasadded LiBOB in an amount of 0.00783 mol (1.52 g), thereby to prepare afinal electrolyte.

Using the members, a cell was formed, and an initial charging treatmentwas performed in the same manner as in Example 1. As a result, the cellin accordance with Example 7 was obtained.

[Measurement of Initial Internal Resistance]

Respective cells were adjusted to a SOC (State of Charge) of 60%. At atemperature of 25° C., respective currents (I) of 0.2 A, 0.4 A, 0.6 A,and 1.2 A were passed therethrough to measure respective cell voltages(V) after 10 seconds. The current value (I) passed through each cell (Xaxis) and the voltage value V (Y axis) were subjected to linearregression. From the slope, the initial IV resistance (mΩ) wasdetermined.

[Charging and Discharging Cycle Test]

For each cell, at 60° C., an operation of performing charging at a rateof 2 C until the inter-terminal voltage becomes 4.1 V, and an operationof performing discharging at the same rate of 2 C from 4.1 V to 3.0 Vare referred as one charging and discharging cycle. This was repeated500 cycles.

[Internal Resistance Increase Rate]

For each cell after the charging and discharging cycle test, the IVresistance (mΩ) at 25° C. was measured in the same manner as with themeasurement of the initial internal resistance. Thus, the internalresistance increase rate (%) was determined as the percentage of the IVresistance value at the time point of completion of the cycle test withrespect to the initial IV resistance value.

For each of the cells 1 to 7, the measured initial internal resistancevalue (mΩ), and internal resistance increase rate (%) are shown togetherwith the total moisture content a (mol), the additive amount β (mol),and the value of log(β/α) in Table 1. Further, the relationships betweenthe log(β/α) and the initial internal resistance value (mΩ) (left Yaxis) and the internal resistance increase rate (%) (right Y axis) areshown in FIG. 4.

TABLE 1 Additive Initial Resistance Total moisture amount log resistanceincrease Ex. content α (mol) β (mol) (β/α) (mΩ) rate (%) 1 0.011110.00060 −1.268 70 110 2 0.00278 0.00090 −0.489 66 104 3 0.00167 0.001960.070 65 103 4 0.00111 0.00392 0.547 66 101 5 0.00067 0.00588 0.945 70102 6 0.02778 0.00030 −1.967 73 140 7 0.00028 0.00783 1.450 82 101

As shown in Table 1 and FIG. 4, for the cells 1 to 5 each with alog(β/α) within the range of −1.3 to +1, all the initial internalresistances were as low as 70 mΩ or less as compared with the cells 6and 7 each with a log(β/α) outside the range. Further, the cells 1 to 5were lower in internal resistance increase rate than the cell 6 by asmuch as about 30 to 40%. Out of these, for each of the cells 2 to 4 witha log(β/α) within the range of −0.5 to +0.6, the initial internalresistance was about 65 mΩ, and the internal resistance increase ratewas 105% or less. Thus, the initial internal resistance and theendurance resistance increase rate were both reduced, and the internalresistance suppressing effect was implemented.

Up to this point, specific examples of the present invention weredescribed in details. However, these are merely examples, and do notlimit the scope of the appended claims. The technologies described inthe claims include various modifications and changes of the specificexamples exemplified up to this point.

REFERENCE SIGNS LIST

-   1 Vehicle (Car)-   10 Lithium ion secondary cell-   20 Wound electrode body-   30 Positive electrode sheet (positive electrode)-   32 Positive collector-   35 Positive electrode mixture layer-   40 Negative electrode sheet (negative electrode)-   42 Negative collector-   45 Negative electrode mixture layer-   50A, 50B Separator

1.-4. (canceled)
 5. A lithium ion secondary cell, comprising: anelectrode body including a positive electrode having a positiveelectrode active material, a negative electrode having a negativeelectrode active material, and a separator, as constituent members; anda non-aqueous electrolyte containing a lithium salt as a supporting saltin an organic solvent, the electrode body and the non-aqueouselectrolyte being accommodated in a case, the positive electrode activematerial including, as a main component, a layered oxide containinglithium and a transition metal as constituent metal elements, thesupporting salt being a lithium salt containing fluorine as aconstituent atom, and the electrolyte containing, as an additive, acompound represented by the following chemical formula (I):

wherein the cell is configured such that the case accommodates thereinthe additive in a number of moles of β satisfying the followingmathematical expression (A): [Mathematical Expression 1]−1.3≦log(β/α)≦1  (A) where α represents the total number of moles ofmoisture to be mixed in the case when the cell is assembled.
 6. Thelithium ion secondary cell according to claim 5, wherein the mixingmoisture into the case is defined as the moisture brought by thepositive electrode, the negative electrode, the separator and theelectrolyte.
 7. The lithium ion secondary cell according to claim 5,wherein the supporting salt is at least one selected from LiPF₆, LiBF₄,LiAsF₆, and LiSbF₆.
 8. A method for manufacturing the lithium ionsecondary cell according to claim 5, comprising: (a) a step of preparingthe positive electrode, the negative electrode, the separator, and aninitial electrolyte serving as constituent members of the cell, theinitial electrolyte including, as a supporting salt, a lithium saltcontaining fluorine as a constituent atom, and the supporting salt beingdissolved in an organic solvent in a prescribed concentration; (b) astep of ascertaining, based on the content of moisture contained in thepositive electrode, the negative electrode, the separator, and theinitial electrolyte prepared in the step (a), a molar equivalent value αof the total content of moisture to be mixed in the case due to theconstituent members; (c) a step of substituting a value of the α intothe mathematical expression (A), and calculating a numerical value rangeof the β satisfying the mathematical expression (A); (d) a step ofadding the additive in a number of moles within the numerical valuerange of β calculated in the step (c) to the initial electrolyte, andpreparing a final electrolyte; and (e) a step of assembling a lithiumion secondary cell under an environment with a dew point of −10° C. orless using the positive electrode, the negative electrode, theseparator, and the final electrolyte.
 9. An assembled battery includinga plurality of lithium ion secondary cells according to claim 5connected in series and/or in parallel.
 10. An assembled batteryincluding a plurality of lithium ion secondary cells according to claim6 connected in series and/or in parallel.
 11. A vehicle comprising thelithium ion secondary cell according to claim
 5. 12. A vehiclecomprising the lithium ion secondary cell according to claim 6.